Heat the oil in a large saucepan over medium heat. Add the onion, garlic, and anchovies. Cover and simmer until the garlic begins to take on color, about 3 minutes. Add the tuna fish, parsley, and hot pepper flakes to taste. Cook for several minutes, breaking up the tuna with a wooden spoon. Add the tomatoes, cover, and simmer for 20 to 30 minutes, until the sauce is the consistency of a marinara (all the tomato should be cooked down). If the sauce is dry, add a little water to loosen it up. Add salt to taste.

Bring a large pot of salted water to a boil over high heat and add the spaghetti. Cook until it is al dente. Drain and add to the sauce. Toss well and serve.